1. Field of the Invention
The present invention relates to an optical system controlling apparatus and, more particularly, to an arrangement suitable for use as, for example, a lens position controlling apparatus for adjusting focus or varying magnification by moving lenses.
2. Description of the Related Art
The recent trend of cameras which are desired to have distinctive performance and miniature size is the wide use of a so-called inner focus lens arranged to adjust focus by moving a rear group of lenses.
FIG. 1 schematically shows one example of a lens construction used in the inner focus type of lens system. The shown lens construction includes a fixed first lens 101, a second lens (zooming lens) 102 for varying magnification, an iris 103, a fixed third lens 104, a fourth lens (focusing lens) 105 which performs both a focus adjusting function and the function of correcting the movement of a focal plane resulting from the magnification varying operation of the second lens 102, and an image plane 114.
FIG. 2 is a graphic representation using a subject distance as a parameter, and shows the loci of the fourth lens 105 for forming an in-focus image in the image plane 114, the fourth lens 105 being shown as moving with respect to variations in the focal length in the case of the inner focus type of lens system which is placed under electronic control. Since the fourth lens 105, as described above, performs both the focus adjusting function and the function of correcting the movement of the focal plane resulting from the magnification varying operation, the movement of the fourth lens 105 draws correction curves unique to individual subject distances. In other words, when zooming is to be performed, it is necessary to select one correction curve corresponding to a subject distance of interest from those shown in FIG. 2 and cause the fourth lens 105 to move in accordance with the selected correction curve.
A method of causing the lens 105 of FIG. 1 to move along a correction locus unique to a particular subject distance is proposed in, for example, Japanese Laid-open Patent Application No. Hei 1-280709.
In this method, the loci shown in FIG. 2 are divided into zones each including a group of loci drawn at an approximately equal inclination as shown in FIG. 3, and one representative speed is assigned to each of the zones. Before zooming, if the fourth lens 105 is located in an in-focus position with respect to a subject, a specific zone is determined from among the zones of FIG. 3 in accordance with the positions of the zooming and focusing lenses. At the same time that zooming is started, the representative speed of the fourth lens to be driven, i.e., the focusing lens 105, is determined. After the zooming has been started, the focal length and the position of the focusing lens 105 vary progressively. As a result, as shown in FIG. 3, the focusing lens 105 progressively moves from one zone to another and its representative speed also changes progressively.
By connecting the steps of displacement corresponding to the changes in the representative speed, it is possible to obtain curves which approximate those shown in FIG. 2, whereby the zooming can be performed with an in-focus state maintained.
However, the above-described method has the problem that the representative speed for each of the zones is determined with respect to a single zooming-lens moving speed and if the zooming-lens moving speed varies due to, for example, a variation in the torque of a zooming motor, a temperature change or a change in the attitude of a camera, the focusing lens does not correctly follow any of the loci of FIG. 2 and defocus occurs during zooming.
Japanese Laid-open Patent Application No. Hei 1-319717 proposes a method of adjusting a zooming-lens driving speed during zooming by increasing or decreasing a coefficient to be multiplied by the aforesaid representative speed in accordance with a change in an actual zooming speed.
Referring to FIG. 3, for example, the horizontal axis is divided into 16 equal parts. If it is assumed that the speed of the zooming lens is set to a speed which permits the zooming lens to move between a telephoto end and a wide-angle end in 7 seconds, 26 vertical sync periods (26 Vsync) are required for the zooming lens to pass through a single zone 701 as shown in FIG. 4 in the case of the NTSC system. If N (Vsync) is taken to pass through the single zone during actual zooming, the change ratio RZS of the actual zooming speed to a reference value (7 seconds from the telephoto end to the wide-angle end) of the zooming speed is expressed as: EQU RZS=N/26 (1)
Accordingly, during zooming, by always measuring the number of vertical sync periods required to pass through the aforesaid single zone and multiplying 1/RZS by the aforesaid representative speed, it is possible to perform the zooming at the moving speed of the fourth lens 105 according to a variation of the zooming speed without defocus.
If the first zooming is to be performed after the power source has been turned on, there is no measurement data as to zooming speeds and RZS is not updated with a correct value obtained from the actual driving speed of the zooming lens. To cope with this problem, the present applicant has developed a system which includes the steps of causing the zooming lens to move by the minimum amount required for measurement immediately after the power source has been turned on, determining the initial value of RZS, finding an actual focusing-lens speed by multiplying this value of RZS by a representative speed corresponding to the zone where the zooming lens is located, then returning the zooming lens to its original position, and subsequently establishing ordinary operating conditions.
Position control over the fourth lens, i.e., the focusing lens 105, will be explained below.
As is apparent from FIGS. 2 and 3 as well as the foregoing description, drive control over the fourth lens requires accurate position detection and good speed switching performance. To meet the requirement, a stepping motor or a similar device which is not easily influenced by inertia and which provides a constant amount of drive with respect to a drive signal and has a wide speed response range, is widely used as an actuator which constitutes a fourth-lens driving means. If a stepping motor is used as an actuator for the focusing lens 105, it is convenient to utilize a position detection method including the steps of counting the number of drive pulses outputted from the stepping motor by using a counter and causing the count to correspond to the position of the focusing lens. In this method, the counter serving as a position encoder is an incremental counter, and to cause the count to correctly correspond to the coordinate of the vertical axis of the locus diagram of FIG. 3, from the time the power source is turned on until the time an ordinary photographic operation is started, it is necessary to execute control to move the focusing lens 105 to a predetermined position, substitute a value corresponding to the predetermined position into the counter with the focusing lens 105 located at that position, and start counting for detecting the position of the focusing lens 105 which varies with the movement thereof.
To realize such control, in the above-described system, it is necessary to perform measurement of the zooming speed and presetting of the position detection counter for the fourth lens 105 between the time the power source is turned on and the time the ordinary photographic operation is started.
However, the above-described arrangement has contradictory problems. Since plural kinds of preparatory operations must be performed within a short time between the time the power source is turned on and the time the ordinary photographic operation is started, it takes a long time to effect a reset operation. As a result, a photographer must wait for a long time until the system starts the ordinary photographic operation. If a reset time is limited so as to reduce the waiting time of the photographer, imperfect resetting will take place and a good ordinary photographic operation will not be performed due to variations in the state of the lens system immediately after the power source is turned on.